Inhalation challenge with the culture filtrate-antigens prepared from two strains of different serotype of Trichosporon cutaneum (TIMM 1573, serotype I; TIMM 1318, serotype II) was performed on patients with summer-type hypersensitivity pneumonitis and asymptomatic seropositive family members. Of the 17 patients, 12 were strongly positive, four were mildly so, and one was negative. Interestingly, of the 16 inhalation challenge-positive patients, four reacted to both serotypes I and II, five to serotype I only, and seven to serotype II only. There was a good correlation between the serotype inhalation challenge-positive antigen and that of T. cutaneum isolated from the homes of two patients' homes. A strain of T. cutaneum demonstrating a new serotype was isolated from the homes of two patients, one of whom was negative to both serotypes I and II. Specific antibody activity, lymphocyte proliferative response, and skin reaction to the antigens were also positive, but these findings were not useful to discriminate the inhalation challenge-positive from the inhalation challenge-negative antigen. Neither of the two asymptomatic family members responded. These results indicate that inhalation challenge in patients with summer-type hypersensitivity pneumonitis is provoked by the serotype-related antigen of T. cutaneum, reflecting the sensitization of these patients in their homes.